Horizontal directional drills or boring machines may be used to install or replace underground utilities with minimal surface disruption. Horizontal directional drills may utilize single member drill strings or dual-member drill strings to create the desired borehole. Drills that use dual-member drill strings are generally considered “all-terrain” machines because they are capable of drilling through soft soil as well as rocks and rocky soil. Dual-member drill strings comprise a plurality of dual-member pipe assemblies. Each dual-member pipe assembly has an inner member supported inside an outer member. The inner member is generally rotatable independent of the outer member. The inner member may be used to rotate a boring tool supported at the end of the drill string. As used herein “boring tool” means the drill bit and housing used to support the drill bit. Such housing may be configured to support a drive shaft and a beacon. The drive shaft may be configured to connect the inner member of the drill string to drive rotation of the drill bit.
In large diameter drilling operations the inner member may be a tubular pipe section with hex ends welded to each end. However, in small diameter drilling operations the inner member must be a solid rod because of space constraints and to handle the torque and thrust forces exerted on the inner member during drilling.
The dual-member drill string permits selective rotation of the outer member to align and hold a steering feature used to change the direction of the borehole while the rotating drill bit continues to drill. One such system is described in U.S. Pat. No. 5,490,569, entitled Directional Boring Head with Deflection Shoe, the contents of which are incorporated herein by reference.
All-terrain, dual-member drill string systems have been very effective for drilling in various soil conditions. However, there are significant stresses placed on the dual-member drill string and its various components during drilling. There is also a general desire to deliver more drilling fluid to the boring tool in small diameter operations to improve cooling the boring tools and float cuttings to the surface. The dual-member pipe assembly of the present invention provides a pipe assembly that has replaceable component parts and a hollow inner member that may be used in small diameter drilling operations. The hollow inner member provides increased fluid flow to the boring tool and improved performance and durability of each pipe assembly.